WHY?

If you right-click on a scroll bar in WinXP, you get this little menu.

What possible use is it? Everything else is taken care of other ways. Scroll here? Drag the box here. Top or bottom? Drag the box. Page up/page down? Click above or below the box. Just scroll up or down? Use the arrows, stupid!

Even better, use the keyboard shortcuts for any of this. You have home and end keys, you have page up and down, and even arrow keys. They were made to do the exact same task...